A fuel vapor treatment system is well known. In the fuel vapor treatment system, fuel vapor generated in a fuel tank is temporarily adsorbed by a canister, and a mixture gas of desorbed fuel vapor and air is purged into an internal combustion engine to be combusted. JP-6-101534A shows a fuel vapor treatment system in which a fuel vapor concentration of the mixture gas purged through a purge passage is detected as a fuel vapor condition quantity such that an exhaust gas air-fuel ratio during purging is precisely controlled.
In the fuel vapor treatment system shown in JP-6-101534A, when the mixture gas flows through the purge passage, the fuel vapor concentration can be detected. Hence, in order that the detected fuel vapor concentration is reflected to the air-fuel control from a beginning of starting the purge, it is necessary that the fuel vapor concentration is detected before the purged mixture gas reaches a fuel injection position of the engine. However, in a case that a volume of an intake passage from an outlet of the purge passage to the fuel injection position is small, or in a case that velocity of intake air in the intake passage is high, the mixture gas reaches the fuel injection position before the fuel vapor concentration is detected. This deteriorates an accuracy of air-fuel control.
JP-2006-161795A (U.S. Pat. No. 6,971,375B2) shows a fuel vapor treatment system in which fuel vapor desorbed from the adsorbent is mixed with air, and the mixture gas is introduced into a detection passage to detect a fuel vapor concentration of the mixture gas. In this fuel vapor treatment system, since the fuel vapor concentration is detected before starting the purge, a large quantity of purge can be achieved by reflecting the detected result to the air-fuel ratio control from a beginning of the purge.
In the fuel vapor system shown in JP-2006-161795A, the fuel vapor concentration of the mixture gas which desorbed from the canister is detected in a detection passage connected to the purge passage prior to the purge. Hence, it is difficult to accurately detect the fuel vapor concentration of the mixture gas remaining in a downstream portion of the purge passage before the purge. After a large quantity of fuel vapor is adsorbed while the purge is stopped due to a fuel supply or a long-term parking, a fuel vapor detection concentration and an actual fuel vapor concentration of the mixture gas remaining in the purge passage deviate from each other. Thus, while the remaining mixture gas in the purge passage reaches the fuel injection position, it is relatively difficult to control the exhaust gas air-fuel ratio accurately.